316 Lieut.- Gen. McMaJion — Tourmaline of White Granite. 



minerals of more than one famil}', which will rarely be the case, 

 the rule is to choose those which are most abundant. The family- 

 name being thus obtained, observe next upon what basis the family 

 is subdivided, whether it is by the nature of the ferromagnesian 

 mineral, by the kind or quantity of felspar present, or by some 

 other property. All the constituents of the rock, essential and 

 accessory, being now considered, it will be easy to assign the rock 

 to its right subdivision, supposing one already exists. If, on the 

 other hand, the rock is a new type and no place awaits it, a new 

 subdivision can be created and named in accordance with the 

 general system. 



The only rocks which it will be found difficult to name are those 

 which do not contain the index minerals of any family. In such 

 cases the association of the rock is the first point to be considered. 

 If any such rock clearly forms part of an igneous mass which can be 

 readily assigned to a family, let the doubtful rock also be assigned 

 to the same family, and give it a name in which the missing index 

 mineral is preceded by the privative a-. If, however, besides being 

 entirely new, the rock does not appear to be associated with any 

 known type, it becomes a question whether a new family should 

 be established. This course must be adopted only when there is 

 really no other way of naming the rock, and the family name 

 selected should be, as far as possible, descriptive of the rock's 

 peculiarity in mineral or chemical composition. 



I cannot end without expressing my gratitude to Prof. Bonney, 

 my first master, for the interest with which he has followed my work 

 upon classification and nomenclature and for the many suggestions 

 which he has freely offered. To Prof. Eosenbusch also I am deeply 

 indebted for the kindly instruction received at Heidelberg, which 

 has opened my eyes and smoothed my path in many ways, but more 

 especially in this research. 



III. — Notes on the Tourmaline of the White Geanite of 

 Meldon, Dartmoor. 



By Lieutenant- General C. A. McMahon, F.R.S. 



fjlHIS "remarkable variety of granite" was briefly described by 

 i Mr. J. J. Harris Teall, F.R.S., in his " British Petrography " 

 (1888), p. 316, and an interesting account was given in a footnote 

 of the process by which the author was able to identify the topaz 

 found in the rock. 



In a paper on Dartmoor published in 1893 (Q.J.Gr.S., vol. xlix, 

 p. 385), I described in some detail the mode of occurrence of this 

 intrusive rock and some of its characteristics ; and in the following 

 year (Q.J.G.S., vol. l, p. 338) I noted the occurrence of a second 

 outcrop on the flank of South Down. 



None of the above references to the white granite of Meldon 

 ■contain a description of the tourmaline found in it, and as this 

 mineral presents some unusual features a few remarks on it may 

 interest students of petrology. 



